Nuclear Magnetic Resonance (NMR) apparatuses are becoming increasingly important in standard clinical routines. They make it possible, in particular, to display virtually all organs of a human body. Such apparatuses use a static magnetic field created by a magnet whose technology depends primarily on a desired intensity. Magnetic fields created by coils having magnetic field gradients make it possible to locate all points of a volume in space. Radio frequency antennas permit excitation and reception of an NMR signal. Devices combining the above fields and signals make it possible to obtain anatomical images with a spatial resolution of the order of 0.5 mm.
The practical development of these devices has been led mainly from "whole body" apparatuses into which a patient is introduced. A zone of interest to be examined is then positioned at a center of the apparatus which surrounds the patient.
More recently, a new type of machine has appeared which no longer examines the whole of the human body but only specific regions. In this case, a system for generating magnetic fields generally has a limited diameter of access, preventing the entire patient from being introduced, but being nevertheless sufficient for introducing the zone of interest. Such a device is described, for example, in EP-A-176 353. These dedicated machines are used for examining specific zones of relatively small size (knee, wrist, breast etc.). Nevertheless, they cannot be used when it is desired to observe a small zone on an object, or on a body of a greater size, as for example when one wishes to examine a mole on the patient's shoulder. In this case, it is necessary to introduce the patient as a whole into a "whole body" device.
In the two approaches discussed above, a size of systems for generating a main magnetic field, and the magnetic field gradients, as well as a radio frequency transmission/reception system, is directly related to the volume of the patient or of the zone of interest to be examined.
U.S. Pat. No. 4,870,363 describes an apparatus whose system for creating a magnetic field gradient along one spatial direction is entirely situated on a same side of an open surface, while a body to be examined is situated on another side of the open surface. Such a system is integrated into a device of the "whole body" type, wherein there are generated the main magnetic field, and the field gradients in the two other directions, as well as the radio frequency transmission/reception.
Moreover, all the devices referred to above have the drawback of being heavy, bulky and very expensive.
EP-A-0 512 345 describes a method and an apparatus for obtaining imaging by high-resolution and high-speed magnetic resonance. Typically such a device comprises a set of coils for generating a magnetic field, a radio frequency transmission/reception system, and a system of field gradients in three orthogonal directions, all these elements being disposed on the same side of the surface to be examined, in this case the surface of the earth. According to this document, the magnetic field is perpendicular to the surface examined. Such a configuration results in a limited potential of the system, in particular in terms of the size of the device. This is particularly the case when the device has to be used in a doctor's surgery, for example. Besides, the signal measurement is particularly complicated due to a geometry of the reception system.
U.S. Pat. No. 5,390,673 describes a device for imaging by magnetic resonance that suffers mainly from a drawback in that a field gradient is created in only one direction of space. In fact, such a system makes it possible to obtain surface imaging from a non-homogeneous field derived from a homogeneous field combined with a natural gradient in the direction perpendicular to the surface to be examined. The scope of such a system is limited, particularly in terms of image resolution. Moreover, like the device of the preceding document, the magnetic field is perpendicular to the surface to be examined.
Apart from the fact that it uses a magnetic field perpendicular to the surface to be examined, the device described in EP-A-0 186 998 uses the same means for generating the homogeneous magnetic field, as well as the field gradients. With such a device, it is particularly difficult to undertake high-resolution imaging. Moreover, it is also extremely difficult to create field gradients in the three orthogonal spatial directions and to switch over quickly.
In U.S. Pat. No. 4,721,914, the transmission/reception system is constituted by a coil disposed all round the patient's head. However, the device has no magnetic field gradients, and such a device is limited to use in localized spectroscopy.